How to fuse/weld/combine 2 "tunnles" in scuplting
Fastest way to solve this is:

Go to edit mode. Select the areas oposing each other (probably with C - circle selection).

Run Edge - bridge edge loops. 

I assume you are using Dyntopo - dynamic topology, so then you can return to sculpt mode, and with dyntopo on, you can add resolution and smooth this area.